Hitherto, a container, a molded product, or the like formed of a metallic can, a glass bottle, or a thermoplastic resin has been used as a packaging material for, for example, drugs, beverages, foods, or chemicals. A container or molded product formed of a thermoplastic resin is widely used, since it is excellent in terms of light weight, moldability, packageability such as sealability, and cost. Among thermoplastic resins, a polyester mainly having ethylene terephthalate repeating units (hereinafter the polyester may be referred to as “PET”) exhibits excellent mechanical properties, melt stability, solvent resistance, smell-retaining property, transparency, recyclability, etc., and thus is widely used for producing, for example, films, sheets, and hollow containers. Also, since PET exhibits relatively high oxygen barrier property, when PET is applied to a beverage bottle which is refrigerated, the content of the PET bottle can be stored for a sufficient period of time.
However, in recent years, demand has arisen for reducing the environmental burden due to mass production of plastics and increased waste. Thus, the thickness or weight of a container formed of PET has tended to be reduced. When the thickness or weight of a container formed of PET is reduced, the oxygen barrier property of the container may be deteriorated. Therefore, there has been required a technique for enhancing the oxygen barrier property of such a PET container, in order to reduce the thickness or weight of the container while maintaining its storage ability.
In a known technique for suppressing permeation of oxygen from outside a container formed of PET, PET is employed in combination with a material exhibiting oxygen barrier property superior to that of PET. For example, there have been proposed a method for producing a container having a multi-layer structure including a PET layer, and at least one layer which exhibits excellent oxygen barrier property and is formed of, for example, poly-m-xylylene adipamide, an ethylene-vinyl alcohol copolymer, polyacrylonitrile, or aluminum foil; and a method for producing a container from a layer formed through melt-blending of PET with the aforementioned thermoplastic resin exhibiting excellent oxygen barrier property (see Patent Documents 1 and 2). However, a container produced through such a method cannot completely block permeation of a very small amount of oxygen from outside the container. In addition, degradation of the content of the container, which may be due to oxygen remaining in the container, cannot be prevented.
Also, there has been proposed a method in which the interior or exterior of a container formed of PET is coated with a thermosetting resin exhibiting excellent oxygen barrier property, or is vapor-deposited with an inorganic oxide (see Patent Documents 3 and 4). However, a container produced through such a method also cannot completely block permeation of a very small amount of oxygen from outside the container. In addition, when impact or an external force such as folding force is applied to the container, cracking may occur in a thermosetting rein film or an inorganic-oxide-deposited film, resulting in a problem in that oxygen barrier property is considerably lowered.
In a known method for completely blocking permeation of a very small amount of oxygen from outside a container, the container itself is imparted with an oxygen-absorbing function. In this method, permeation of oxygen from outside the container can be prevented, and oxygen remaining in the container can be removed. Therefore, the storage ability of the content of the container can be considerably improved.
Means for achieving this method is to add an oxygen-absorbing agent to a thermoplastic resin forming the container. For example, Patent Document 5 or 6 describes an oxygen-absorbing layered product or oxygen-absorbing film formed of a resin in which an oxygen-absorbing agent containing iron powder, etc. is dispersed.
Meanwhile, Patent Document 7 describes an oxygen-scavenging wall for packaging, in which a polyamide containing a metal catalyst such as cobalt serves as an oxygen-absorbing material for absorbing oxygen present inside and outside of a container. Patent Document 8 describes a container including an oxygen-absorbing layer formed of a gas-barrier resin containing an ethylenic unsaturated compound such as polybutadiene and a transition metal catalyst such as cobalt. Also, there has been proposed a method in which a container is produced from oxygen-absorbing PET formed through copolymerization of PET with a compound having an oxygen-absorbing function. For example, Patent Document 9 describes a container formed of a copolyester having a polyester segment and a polyolefin segment exhibiting oxygen-scavenging property.